下一代WEB访问模型

本文尝试性地从代数结构的角度观察在新技术广泛应用的条件下Web访问模式的演变,详细探诗了第一代Web的访问模型及其优缺点,并在一定合理假设的前提下提出了第二代Web的访问模型,同时讨论模型的一些有趣性质,为研究IT产业链提供依据,为网络访问模式的深入研究奠定基础。     

分布式多媒体系统中对象表示模型及变换方法研究

在设计一个分布式多媒体系统时，具有挑战性的问题之一是如何在系统的不同层次为各模式设计合适的规格描述模型，并提供不同模型的变换方法的集成与同步策略，在本文中，我们给出了几种多媒体对象的表示模型和变换算法，它们把高层的多媒体对象变换成可以用来支持多媒体对象展示和通信的实用模式，在该模式中能较好地保持多媒体对象的集成与同步特性。     

Tree Rerooting in Distributed Garbage Collection: Implementation and Performance Evaluation

We have recently defined a new algorithm for distributed garbage collection based on reference-counting (Luc Moreau, in Proceedings of the Third International Conference of Functional Programming (ICFP'98), Sept. 1998, pp. 204–215; Luc Moreau and J. Duprat, Technical Report RR1999-18, Ecole Normale Supérieure, Lyon, March 1999). At the heart of the algorithm, we find tree rerooting, a mechanism able to reduce third-party dependencies by reorganising diffusion trees. In reality, the algorithm describes a spectrum of algorithms according to the policy used to manage messages. In this paper, we present the implementation of the algorithm and evaluate its performance. We have implemented two policies, which are extremes of the spectrum, respectively using and not using tree rerooting. In addition, two different strategies for managing action queues have been implemented. The conclusions of our experimentations are the following. Tree rerooting offers more parallelism during distributed GC activity; we explain this phenomenon by the length reduction of causality chains in the distributed GC. Grouping messages per destination dramatically reduces the number of messages, but requires a more complex implementation as messages have to be sorted per destination. Speed up of 100% has been observed on some benchmarks.     

Experimental Analysis of Timing Validation Methods for Distributed Real-Time Systems

Scheduling jobs dynamically on processors is likely to achieve better performance in multiprocessor and distributed real-time systems. Exhaustive methods for determining whether all jobs complete by their deadlines, in systems that use modern priority-driven scheduling strategies, are often infeasible or unreliable since the execution time of each job may vary. We previously published research results on finding worst-case bounds and efficient algorithms for validating systems in which independent jobs have arbitrary release times and deadlines, and are scheduled on processors dynamically in a priority-driven manner. An efficient method has been proposed to determine how late the completion times of jobs can be in dynamic systems where the jobs are preemptable and nonmigratable. This paper further presents the performance characteristics of the proposed methods, and shows its soundness by providing extensive simulation results. The worst-case completion times of jobs obtained with the proposed methods are compared with the values by simulations under different workload characteristics. The simulation results show that the proposed algorithm performs considerably well for diverse workloads. Considering the previous work showed the unlikelihood of finding tighter bounds than the one given in the paper, the simulation results indicate that the proposed methods effectively constitute a theoretical basis needed for a comprehensive validation strategy that is capable of dealing with dynamic distributed real-time systems.     

一种分布式视频点播系统模型

视频点播（Video-on-Demand）是现代网络上的一个重要多媒体应用，在传统的集中式视频点播系统中，如何用户数目过多，并可能出现服务器I／O及网络带宽的瓶颈，而采用发布式体系结构的VOD系统可以将负载分布到放置在网络不同位置的各个服务器上，能很好地解决系统瓶问题，本文提出了一种分布式视频点播系统模型，包括系统结构、软件结构、功能定义及实现上的考虑。     

A Real-Time Performance Evaluation Model for Distributed Software with Reliability Constrains

In this paper, we propose an approach for the real-time performance analysis of distributed software with reliability constraints, called Athena. The approach is based on the real-time and reliability performance analysis of distributed program. In Athena, two important factors, imperfect nodes and the links reliability, are introduced. The algorithms proposed in Athena generates sub-graphs, counts the reliability of each sub-graph, calculates the transmission time for all the transmission paths of each data file, and computes response time of each data file with reliability constraint. In this way, the real-time performance of distributed software with reliability constrains can be evaluated.This paper is supported by National Science Foundation of China under grant 60273076.     

基于异构分布式系统的实时容错调度算法

目前文献中研究的实时容错调度算法都是基于同构分布式系统,系统中的所有处理机完全相同。该文首先建立了一个基于异构分布式系统实时容错调度模型,异构分布式系统中的各个处理机均不相同。基于该异构分布式系统模型,该文引入了可靠性代价（reliability cost）概念,并提出两种静态实时容错调度算法（RTFTNO和RTFTRC）用于调度周期性实时容错任务。算法RTFTRC在调度任务时,尽量使系统的可靠性代价最小;而算法RTFTNO在调度实时任务时,没有考虑系统的可靠性代价。该文详细讨论了两种调度算法的性能。性能模拟实验分别比较了两个算法的可靠性代价,超时比率和可调度性;并研究了任务的计算时间与可靠性代价的关系以及调度长度阈值与最小处理机个数的关系。实验结果表明,算法RTFTRC的性能优于算法RTFTNO。     

基于时间序列分析的分布式拒绝服务攻击检测

该文分析了分布式拒绝服务(DDoS)攻击的特点,提出了一种基于流连接密度(FCD)时间序列分析的DDoS攻击检测方法,该方法通过拟合FCD时间序列的自适应自回归模型,获得能够在多维空间描述当前流量状态的AAR模型参数向量序列,然后使用经过样本训练的支持向量机(SVM)分类器进行攻击识别;充分考虑了报警的时间间隔及分布情况,提出一种报警可信度评估算法对SVM分类结果进行二次处理,以消除网络流量噪声及分类错误所带来的影响．实验结果显示,该检测方法能够有效检测DDoS攻击,可信度评估算法能够明显减少误报,降低误报率,显著提高检测质量．     

Web服务系统的性能分析与测试

总结了衡量Web服务性能的重要评价指标,并以Web Server性能测试工具－－SPECweb99为例阐述了测试原理和测试方法。通过性能分析模型和测试实例系统地分析了负载情况、服务器处理能力（着重于动态请求内容的处理方式）、网络带宽等关键因素对Web服务系统性能的影响。这些结论对于提高和改善Web服务性能具有较强的指导意义。     

对等网络Chord分布式查找服务的研究

在大规模Internet网络的应用中,查找服务是对等网络系统所面临的核心问题。Chord提供了一种高效和稳定的查找服务,Chord的可扩展性特点适用于大规模对等网络系统。在节点退出或加入时,Chord提供了高效稳定的机制。基于Chord的文件共享系统(SFS)是一个用于对等网络的只读存贮系统,它可以提供高效率的、鲁棒的和负载平衡的文件存取功能。SFS采用了完全分布式体系结构(没有服务器),很容易地扩展到大规模网络。SFS核心分为二层：DHT层和Chord层,使用Chord算法来定位查找服务。     

面向服务的供应链绩效评价研究

面向服务的架构SOA是一种应用程序体系结构。从国内外供应链管理绩效测评方法研究存在的不足出发,结合供应链绩效管理需求,介绍了SOA定义及其特性,构建了面向服务的供应链绩效管理评价体系架构;描述了其业务应用层、服务总线层及基础服务层的基本组成与功能;分析了该评价体系的运行方式。     

The Design and Performance of a Real-Time CORBA Scheduling Service

There is increasing demandto extend CORBA middleware to support applications with stringentquality of service (QoS) requirements. However, conventionalCORBA middleware does not define standard features to dynamicallyschedule operations for applications that possess deterministicreal-time requirements. This paper presents three contributionsto the study of real-time CORBA operation scheduling strategies.First, we document our evolution from static to dynamic schedulingfor applications with deterministic real-time requirements. Second,we describe the flexible scheduling service framework in ourreal-time CORBA implementation, TAO, which supports core schedulingstrategies efficiently. Third, we present results from empiricalbenchmarks that quantify the behavior of these scheduling strategiesand assess the overhead of dynamic scheduling in TAO. Our empiricalresults using TAO show that dynamic scheduling of CORBA operationscan be deterministic and can achieve acceptable latency for operations,even with moderate levels of queueing.     

基于ICE中间件的分布式搜索服务

分布式计算具有很多的优点,ICE提供了分布式计算的基础环境。基于ICE构建分布式搜索服务．矗用路由配制形式,子搜索逻辑可以动态增删改,搜索规则可以动态定义;同一服务可以动态增减实例,满足动态的性能需求;能够负载均衡和灾难冗余。具有很好可伸缩性、可用性,同时可以节约成本。     

Scheduling Video Streams in Video-on-Demand Systems: A Survey

Developments in multimedia technology over the past decade has caused video-on-demand services to emerge as a new paradigm in home entertainment. Because of the large volume of data involved in the process and stringent continuity and real-time constraints, these services pose challenges that are different from the standard file transfer operations in the network. The necessity of efficient usage of scarce resources like network bandwidth and server capacity (in terms of I/O bandwidth) demands novel and easy-to-use schemes for scheduling continuous video streams. This paper presents an overview of the major scheduling policies that have emerged in the recent past. In particular, the paper provides detailed discussion on policies based on principles of broadcasting, batching, caching, and piggybacking or merging. Policies like look-ahead scheduling schemes that are designed exclusively to provide certain interactive VCR-like control operations are also covered. A conceptual comparison between the various classes of scheduling policies is carried out to identify common threads and key concepts. Performance of these policies in terms of bandwidth demand reduction, customer waiting time reduction, provision of interactive control by the user, and fairness of service are given special emphasis. The paper concludes with a discussion on the possible avenues of further reasearch and development in this potentially interesting and challenging field.     

基于ICE中间件的分布式搜索服务

分布式计算具有很多的优点,ICE提供了分布式计算的基础环境。基于ICE构建分布式搜索服务,采用路由配制形式,子搜索逻辑可以动态增删改,搜索规则可以动态定义;同一服务可以动态增减实例,满足动态的性能需求;能够负载均衡和灾难冗余。具有很好可伸缩性、可用性,同时可以节约成本。     

利用分布式技术构建信息比对服务中心

随着公安各部门信息化的推进,部门之间联合办公的趋势日显突出,同时也要求公安信息系统越来越多地为政府其他部门和社会提供服务。构建信息比对服务中心的目的是,根据客户需求比对各个警种的信息,以挖掘出有价值的信息。文章利用分布式技术构建了信息比对服务中心,实现了实时比对各种预定义的信息和共享警务信息,有效利用了警务信息并提高了工作效率。     

分布式文件存储系统性能测试问题研究

分布式文件存储系统的性能测试不同于普通的文件系统性能测试。首先,需要根据系统支持的操作确定测试的benchmark,然后,需要根据系统的设计规模对多用户并发访问系统的情形进行模拟。在“燕星”分布式文件存储系统的性能测试中,对Andrewbenchmark进行改造,确定了测试的benchmark;基于RMIcallback机制,模拟了500个用户并发访问系统的情形。并将测试结果与NFS进行了对比,结果表明,“燕星”系统具有良好的实用性能。     

基于Web Service的分布式网站内容监控系统设计

基于Web Service技术设计了一个基于互联网的分布式网站内容监控系统,该系统由网站内容监控中心和多个网站内容监控代理组成。监控中心与各监控代理子系统相互之间通过SOAP协议通信并协同工作。基于该系统,能够对分布在互联网上的众多网站的内容进行有效监控。     

分布式通风机性能测定系统在煤矿的应用与分析

采用分布式通风机性能测定系统对东滩煤矿西翼风井2台主通风机进行了性能测定,绘制出了风量-风压曲线、风量-功率曲线和风量-效率曲线,分析了主(备)通风机在不同工况以及不同叶片角度下的通风能力。实际应用表明,该系统现场测定所需时间少,且数据处理高效准确。